Copied from the FAQs on ThemeForest:
http://themeforest.net/item/mingle-multipurpose-wordpress-theme/faq/235056
How do I disable comments?
Go to “Settings > Discussion” and deselect the option for “Allow people to post comments on new articles“. This applies to both pages and posts.
To individually disable comments on a page or post, from the edit screen click the tab at the top right “Screen Options” and check “Discussion”. This adds a box to the edit screen where you can deselect the options for comments and trackbacks.
Note: If you already have the comments enabled on existing pages/posts doing the global change in Discussion will only have an effect on NEW pages/posts created after that point. You need to go in and individually disable comments for each page/post for the existing content. This is how WP is setup and the theme has no control over it.